The invention relates to a solar panel and more particularly to a high efficiency solar panel for converting incident solar light into heat.
In some prior art types of solar panels, the incident light passes through a window to a heat absorber. The window is intended to transmit solar radiation while trapping the re-emitted infrared radiation from the absorber. The window also reduces convection losses using "hot house" and "window pane" techniques. The window sometimes consists of one or more layers of transparent glass or plastic separated by air. The absorber is designed to be heated by absorption of the energy incident from the sun and to transmit this heat to a heat transfer fluid, which is typically water or air. The absorber is insulated to minimize heat loss to the surroundings. Each of these components and more particularly the window and the absorber can be improved by the application of optical techniques.
The window portion should be highly transmitting to light having a wavelength in the 0.3 to 2 micron range. Also the transmission of the incident light should be independent of the angle of incidence or polarization, thus making the solar panel useful even on a cloudy day while avoiding the use of expensive mechanical tracking systems. Infrared re-radiation from the absorbing portion should be trapped within the panel. The window portion should be made of low cost materials without sacrificing sturdiness.
Some of these requirements are contradictory. For example, in order to make the window portion sturdy or in order to make it more effective in trapping infrared re-radiation, some prior art panels are made with relatively thick window portions which have lower light transmitting qualities than relatively thinner sections. Thus, the light transmitting capability is degraded to improve the capability to trap re-emitted infrared radiation. Furthermore, when the window portion is made relatively thick the acceptance angle properties of the window portion may be degraded in some prior art arrangements.
The ideal absorbing portion must have a high absorptivity for light wavelengths in the 0.3 to 2 micron range and a low emissivity for light wavelengths in approximately the 10 micron range. For a black body, the ratio of absorptivity to emissivity is approximately one whereas for a polished metal surface this ratio is approximately three. Ratios on the order of nine have been obtained by coating a thin absorption layer over a reflecting surface. The layer is thick enough to absorb solar radiation but thin compared to the wavelength of the infrared. In prior art devices of this type, however, such coatings tend to be difficult to apply and lose their high absorptivity to emissivity ratio with age. The disadvantages of a simple polished metal surface is that it is highly reflecting with a reflectivity constant of approximately 0.9. Further, design requirements for such absorbing portions are that they should give good contact with the fluid to be heated, be inexpensive to manufacture and should reduce convection losses.